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Why Plastic Mulch?

BLACK, WHITE OR COLORED, PLASTIC MULCH WORKS FOR
VEGETABLES

In the years following its discover in the 1950’s,
plastic mulch took over the vegetable fields of America – and the world –
as growers discovered the wonderful things long strips of plastic could do
when laid down in rows with plants set into slits and holes.

By the 1970’s, growing vegetables over plastic had
become common, and by the end of the last century most of the uses were
pretty well understood.

Still, new uses and surprises come along now and then.

University of Illinois Senior Research Specialist Bill
Shoemaker and Extension Educator Maurice Ogutu maintain test plots at the
St. Charles Horticultural Research Center each year.

“We still find a few surprises” Shoemaker said. “Blue
mulch particularly has really stimulated the growth and productivity of
melons; more than 1 thought it would.”

He’s also working with metalized silver reflective
mulches in growing pumpkins. The reflected light seems to prevent aphids
from seeing the green pumpkin plants. Fewer aphids feeding on the plants
can mean fewer viruses vectored into the plants, extending the vitality of
the vines and increasing pumpkin yields.

A new plastic film called Blockade from Pliant Corp.
provides an effective barrier that keeps methyl bromide in the soil. Mark
Jordan, sales director for agricultural and industrial films for Pliant,
said the film makes more effective use of the fumigant, which saves
dollars for growers and prevents escape of the gas into the atmosphere.

Methyl bromide was identified years ago as a deplete of
ozone in the atmosphere, and by worldwide agreements its use is being
phased out. Growers who can’t get along without it can apply for Continued
Use Exemptions – and efforts to use the gas conservatively and wisely play
a role in their continuing to get them.

Other films also work as a barrier against the escape
of methyl bromide, but Jordan said the Blockade is five to 20 times as
effective as metalized films, and much more grower-friendly-less stiff and
easier to lay tight to the ground.

Much work with plastic films was done in the 1990s at
Penn State University, and research findings can be found at the Center of
Plasticulture’s Web site,
www.plasticulture.org.

A key issue in choosing to use colored mulch is cost,
said Mike Orzolek, director of the center. Black mulch remains far cheaper
because the key colorant carbon black is less costly than dyes and does
double duty because it effectively stops photodecomposition.

It costs about $160 to put down an acre of black
plastic on 6-foot row centers, he said, and about $240 to do it with
colored mulch.

Biodegradable mulch is also making headway, he said.
Despite its higher initial cost-about twice-it is worth at least $100 an
acre more because it eliminates the retrieval and disposal costs of
conventional plastic mulch. You just rototill it into the soil when the
season ends.

“Plastic mulch has become a standard cultural practice
for many vegetable crops”, Shoemaker said. “Since its discovery in the
late ‘50’s, it has developed into a very useful tool for growers”.

“It can provide excellent weed control within the
planted row, which by itself may be worth its price in some cases. It also
holds moisture in the soil (and) it makes drip irrigation more efficient
as it virtually eliminates evaporation of the irrigation water”.

“But the most powerful feature of plastic mulch is the
impact it has on light”.

“Light is responsible for creating both heat and color.
Plastic mulch can impact the development of heat, increasing it or
decreasing it, depending on mulch color, Shoemaker said.

With white or reflective film the light is largely
reflected away with reduced conversion of light to heat. Heat-sensitive
crops such as lettuce can sometimes benefit from this soil-cooling trait,
he said.

Other colors of plastic mulch films have been
investigated for their impact on crops. Each color film reflects the
wave-length that gives it the color we perceive. Green looks green because
it reflects green light, he said.

“These wavelength differences have subtle, and
sometimes profound, impacts on plant growth and development”, Shoemaker
said. “Early research on red films found that they impacted tomatoes by
forcing more compact growth, early flowering and earlier productivity.
Recent research at St. Charles by Maurice Ogutu and me shows that blue
films seem to stimulate productivity in muskmelons”.

Yellow has been shown to attract insects – but that can
be a good idea. Some growers are using yellow-mulched rows to attract
insects to a location where they can be killed, Jordan said, reducing the
need to spray nearby rows of the same crop planted on mulch of other
colors.

In an article on the Center for Plasticulture Web site,
director Orzolek and associate director William J. Lamont Jr. summarize
the state of knowledge about choosing the color of a plastic film, based
on their years of field research at the Horticulture Research Farm in Rock
Springs, PA.

Some generalities that can be made regarding color are
1) silver repels aphids,
2) Blue attracts thrips but has been very effective in greenhouse tomato
production and
3) Yellow attracts insects. There also appears to be some reduction in
disease pressure with crops grown on specific colors.

They recommend the following colors for specific crops:
Tomatoes responded to red mulch compared to black, with an average 12
percent increase in marketable fruit yield and fruit size over a
three-year study. There appeared to be a reduction in early blight in
plants grown on red.

Peppers responded more to silver mulch compared to
black, with an average 20 percent increase in marketable fruit yield and
fruit size over a three-year study. Lowest yield or marketable peppers
were harvested from plants grown on either white or light blue mulch. In
more southern climates, below North Carolina, pepper plants grown on green
IRT had similar marketable fruit yields compared to plants grown on black.

Eggplant appeared to respond more to red mulch compared
to black with an average 12 percent increase in marketable fruit yield
over two years. Greatest response occurred when plants were growing under
temperature and water stress conditions. There may be a varietal response
of eggplant to the use of plastic mulch, they said.

Cantaloupe responded more to green IRT or dark blue
mulch compared to black, with an average 35 percent increase in marketable
fruit yield over three years. Lowest yield of marketable cantaloupe were
harvested from plants grown on either white or black mulch at this
location. In more southern climates, below North Carolina, cantaloupe
response to white or black mulch would be entirely different, they said.

Cucumbers and summer squash seemed to responded more to
dark blue mulch compared to black, with an average 30 percent increase
over three years in marketable fruit yield for cucumbers and 20 percent
for zucchini. Lowest yield were harvested from plants grown on yellow
mulch at the Pennsylvania farm, but, they said, in more southern
climates-below North Carolina – these crops’ response to yellow mulch
might be entirely different.

Onions and potatoes responded to several mulch colors
including red, metalized silver and black compared to no plastic mulch,
with an average 24 percent yield increase in marketable bulb or tuber
yield. There was a significant difference in yield response between
specific onion varieties.

Potatoes grown on the metalized silver mulch showed the
highest marketable tuber yields, the coolest soil temperature and the
least number of Colorado potato beetle adults. However, they said, the
metalized silver mulch can be difficult to lay in the field and obtain a
tight fit over a raised bed. In cool years the metalized silver mulch may
also cause poor plant emergence. There was a significant difference in
yield response between specific potato varieties and mulch color. Use of
black or possibly red plastic mulch will reduce the highest yield of
quality potatoes, they said.